The present invention relates to a method of manufacturing an electrode used for plugs, and in particular, to a manufacturing method of an electrode applied to a spark plug equipped with two electrodes (i.e., a center electrode and an earth electrode) arranged face to face with a predetermined gap therebetween and on both or one of which a noble metal tip is laser-welded at a plurality of spots to the electrode(s).
There is provided a spark plug that has a center electrode on which a tip made from an iridium alloy is welded. Practically, the tip welded on the electrode by radiating a laser beam onto a plurality of spots (for example, 8 spots) on the root of the tip (i.e., on the area located between the center electrode and the tip).
Conventionally, the welding of the tip to the center electrode is carried out spot by spot, which leads to the problem that it takes much welding man-hours, encountering the problem that it is difficult to reduce manufacturing cost of the spark plug.
Further, at each spot that has received the radiation of a laser beam, both of the tip and the electrode partly become melted. Thus, during the radiation of a laser beam (that is, during the laser welding), it is easier that the tip tilts to the electrode on account of a slight disturbance (external force), being apt to cause defective tips.
To solve this problem about the tilt of the tip, the solution is to reduce the output of the laser beam. However, reducing the output will not enable each spot to be welded of the tip and electrode to melt sufficiently in a shorter time, increasing the man-hours. In addition, the possibility that the welding results in failure is raised.
An object of the present invention is to, with due consideration of the above drawbacks, provide a manufacturing method which is able to reduce a manufacturing cost of a spark plug with reduced man-hours (time required for the manufacture).
In order to realize the above object, as one embodiment, the present invention provides a method of manufacturing a plug electrode applied to a spark plug equipped with two electrodes (2, 3) disposed face to face with a specified gap therebetween, a noble metal tip (2a) being laser-welded at a plurality of spots on at least one of the two electrodes, comprising the steps of: placing at least two laser radiation apparatuses of which axes of radiated laser beams are directed in an indirect face-to-face manner at the tip (2a); and driving the two or more laser radiation apparatuses so that at least two of the laser beams are radiated to at least two of the spots to be welded at a substantially simultaneous timing.
Thus, two spots are subject to simultaneously performed laser welding. Compared to the conventional, the laser welding process can be reduced to approximately half of its process.
In addition, the two laser radiation apparatuses (10, 11) are disposed so that the axes of their laser beams cross at the tip (2a) and engage in the simultaneous laser welding of two spots on the tip (2a) and electrode (2). Therefore, if the focus of one of the two laser radiation apparatuses is moved in any way, there is no fear that the laser beam radiated from the moved apparatus impinges onto the other apparatus.
Thus, with the damage of the laser radiation apparatuses prevented without fail, the laser welding process can be shortened to approximately half of its conventional process.
As a second embodiment, the present invention provides a method of manufacturing a plug electrode applied to a spark plug equipped with two electrodes (2, 3) disposed face to face with a specified gap therebetween, a noble metal tip (2a) being laser-welded at a plurality of spots on at least one of the two electrodes, comprising the steps of: placing at least two laser radiation apparatuses (10, 11) disposed at positions mutually shifted by an angular amount of xe2x80x9c(180-360/N)xc2x110xe2x80x9d degrees (N is the number of welded spots) around the tip (2a); and performing welding by using the two or more laser radiation apparatuses (10, 11) so that at least two of the laser beams are radiated to at least two of the spots to be welded at a substantially simultaneous timing.
Like the first embodiment, two spots are also subject to simultaneously performed laser welding. Compared to the conventional, the laser welding process can be reduced to approximately half of its process.
In addition, two spots are subjected to the simultaneous welding by the two laser radiation apparatuses (10, 11) disposed at positions mutually shifted by an angular amount of xe2x80x9c(180-360/N)xc2x110xe2x80x9d degrees (N is the number of welded spots) around the tip (2a). Thus, like the first embodiment, if the focus of one of the two laser radiation apparatuses is moved in any way, there is no fear that the laser beam radiated from the moved apparatus impinges onto the other apparatus.
Thus, with the damage of the laser radiation apparatuses prevented without fail, the laser welding process can be shortened to approximately half of its conventional process.
In the first and second examples, it is preferred that the welding performing step includes a step of performing, in an even-numbered time of welding process, the welding at a spot different from the last spot subject to the welding during an odd-numbered time of welding process following the even-numbered time of welding process, in cases where one process of welding is defined as a process in which the two laser radiation apparatuses (10, 11) engage in simultaneous laser welding at the two spots.
This configuration makes it possible to prevent an unbalance in the distortion in welding, because the welding is carried out such that paired two welded spots constitute the point symmetry of arrangements as a whole. Accordingly, the tip (2a) is prevented from tilting.
As a third embodiment of the present invention, there is provided a method of manufacturing a plug electrode applied to a spark plug equipped with a center electrode (2) and an earth electrode (3) disposed face to face with a specified gap therebetween, a noble metal tip (2a) being laser-welded at a plurality of spots to the center electrode (2), comprising the steps of: placing two laser radiation apparatuses disposed at positions mutually shifted by a specified amount of angle around the tip (2a); and causing the two laser radiation apparatuses (10, 11) to radiate a laser beam respectively toward each spot for laser welding on the basis of a sequence including a period of laser non-radiated time between welding processes in each of which the two laser radiation apparatuses engage in the simultaneous laser welding at the two spots, the period of laser non-radiated time being a period of time to allow only a fraction defective of 0.5 percents or less in cases where an axial center of the tip (2a) tilts to an axial center of the center electrode (2) by an amount of more than 3 degrees is defined as a defective.
Accordingly, with a fraction defective of 0.5 percents or less maintained, the laser welding process can be reduced to approximately half of its conventional process, because two spots on the tip are also subject to simultaneously performed laser welding.
In the third embodiment, it is preferred that the period of laser non-radiated time existing between a first welding process and a second welding process is shorter than the periods of laser non-radiated time existing between processes following the second welding process. This makes it possible to reduce the fraction defective to a smaller amount.
More preferably, the period of laser non-radiated time is assigned to a period of time during which welded parts of the tip (2a) are cooled. This eliminates temperature at the welded spots from rising excessively, thus a tilt of the tip (2a) being prevented. Preferably, the cooling is self-cooling based on air-cooling.
The references enclosed in parentheses in the above constructions correspond to constituents detailed in the following embodiments, but it is not meant that those references do not limit the scope of the present invention.